The present invention relates to a beam former for a ultrasonic diagnostic apparatus which is used for detecting information about the inside of a human body in a medical field.
In general, a ultrasonic diagnostic apparatus normally uses a beam former which operates to take the steps of sensing echo signals from a plurality of elements, multiplying each echo signal by each delay derived from a distance from the corresponding element and adding those echo signals for improving an S/N ratio. The method employed in the beam former is known by the ordinary persons skilled in this art.
As a delay means for the beam former, an LC analog delay line is often used. The analog delay line is limited in light of the frequency characteristics and is so disadvantageous when a long delay time is needed in a high frequency. Further, the analog delay line is also disadvantageous in that the delay times in respective delay lines and between taps are variable and a dynamic focusing operation needs an analog switch, thereby bringing about noises.
To overcome those disadvantages, a beam former using digital delay lines is considered. It is arranged to have an analog-to-digital converter and a memory. For example, a technique has been proposed in "PROCEEDINGS 0F THE IEEE, Vol. 65, June 1977, page 898 to 904". FIG. 1 shows one prior art of a beam former using the digital delay line. In FIG. 1, numerals 101a, 101b, 101c, . . . , 101c denote elements. Numerals 102a, 102b, 102c, . . . , 102n denote variable gain amplifiers. Numerals 103a, 103b, 103c, . . . , 103n denote analog-to-digital converters. Numerals 104a, 104b, 104c, . . . , 104n denote memories (each being simply referred to as memory) which can execute write and read in a simultaneous and asynchronous manner. Numerals 105a, 105b, 105c, . . . , 105n denote digital-to-analog converters. A numeral 106 denotes a timing control circuit. A numeral 107 denotes an analog adder. A numeral 108 denotes a low-pass filter. Hereafter, the description will be oriented to how this type of beam former operates.
An electric pulse, which is produced by a pulse generator (not shown), is applied to the elements 101a, 101b, 101c, . . . , 101n in which it is converted into ultrasonic sounds. Those elements radiate the ultrasounds toward the inside of a human body. The radiated ultrasounds are reflected on various parts of the human body having respective acoustic impedances and returned to the elements 101a, 101b, 101c, . . . , 101n. Those elements convert the reflected ultrasonic sounds into the corresponding electric echo signals. Those echo signals are properly amplified in the corresponding variable gain amplifiers 102a, 102b, 102c, . . . , 103n and then converted into digital signals in the corresponding analog-to-digital converters 103a, 103b, 103c, . . . , 103n. The digitized echo signals are written in the memories 104a, 104b, 104c, . . . , 104n. Since those memories enable to read and write data simultaneously and asynchronously, they are reading out those signals as they are writing the signals. The signals read out by the memories are sent to the digital-to-analog converters 105a, 105b, 105c, . . . , 105n, from which the converted analog signals are output. As such, the delay time can be arbitrarily controlled in accordance with the operation of the timing control circuit 106 which controls conversion timings of the analog-to-digital converters and the digital-to-analog converters. In the above-mentioned operation, the echo signals provided by the elements 101a, 101b, 101c, . . . , 101n are respectively delayed. The respective delayed signals are added in the analog adder 107. The output of the analog adder 107 is arranged to pass through a low-pass filter 108 so as to cut from harmonics components generated by the conversion of the analog signals to the digital signals executed in the analog-to-digital converters 103a, 103b, 103c, . . . , 103n.
FIG. 2 shows another prior art of a beam former using the digital delay line. In FIG. 2, numerals 101a, 101b, 101c, . . . , 101n denote elements, Numerals 102a, 102b, 102c, . . . , 102n denote variable gain amplifiers. Numerals 103a, 103b, 103c, . . . , 103c denote analog-to-digital converters. Numerals 104a, 104b, 104c, . . . , 104n denote memories which can read and write data simultaneously and asynchronously. Numerals 115a, 115b, 115c, . . . , 115p denote digital adders. A numeral 106 denotes a timing control circuit. Those digital adders 115a, 115b , . . . , 115p are arranged in a so-called "tournament" format. Unlike the above-mentioned prior art, this beam former is arranged to add the digital signals by using the digital adders without having to use the digital-to-analog converters.
The echoes are reflected from various tissues of a human body. Hence, those echoes are respective in intensity. For example, the echo reflected from the diaphragmata is relatively strong, while the echo reflected from blood flow is relatively weak. Considering those various echoes, a Doppler blood flow meter, for example, needs to secure such a large dynamic range as receiving the weak echo from blood flow without having to saturate the strong echo. For securing a wider dynamic range, the digital beam former needs analog-to-digital converters having more bits to be processed. Unfortunately, however, as the number of bits processed in the analog-to-digital converter is increased, the converter becomes more costly.
The cost of the digital beam former for the ultrasonic diagnostic apparatus depends on the cost of the used analog-to-digital converters. Those converters, in general, operate in the range of several MHz to several tens MHz and process 8- to 12-bit data. Such a digital-to-analog converter may often be made several to several tens times more costly even if the number of bits processed therein is increased by only one. That is, rather than increasing the bits to be processed in the converter by one, the addition of one same converter lowers the cost of the digital beam former.